Infrared light transmitting fibers made of a single crystal or polycrystalline metal halide are known, but they generally have a lower break strength than quartz glass fibers. In the course of studies on a method for improving the mechanical strength of the fibers, we came to realize that since extrusion of a metal halide crystal produces a linear polycrystalline element, the metal halide fiber might be reinforced by reducing the size of crystal grains in the same manner as metals can be reinforced by reducing the size of its crystal grains. A silver halide or thallium halide crystal is ductile, but to provide a smooth surface and reduce the energy required for the subsequent working, it is hot extruded into a linear form. The cyrstal undergoes slip deformation into a polycrystalline product, but at the same time, large crystal grains are formed due to recrystallization. In addition, an alkali metal halide which is brittle at ordinary temperatures must be hot worked at elevated temperatures where plastic deformation occurs. The fiber so prepared consisted of large crystal grains having a size between 400 microns and 1 mm and had substantially the same mechanical properties as those of the single crystal, and hence the object of improving the mechanical properties of the fiber by reducing the size of its crystal grains could not be attained.